[heycam/webidl] Add an example of subclassing + [Global] (#748)

In <https://github.com/w3c/css-houdini-drafts/issues/907> it was pointed out that we're incorrectly writing our `Exposed`/`Global` annotations for worklets. In order to understand the right way to do this, I've had to reread the sections on both of those annotations multiple times; even then, it just helps me understand what bz is telling us to do, but I'd still be incapable of figuring that out on my own.

So, it would be cool to have an example of how to set this case up.  I don't have the brains to do fluff text at the moment, but the idea is to illustrate the difference between:

```
[Exposed=MyGlobal,
 Global=MyGlobal]
interface MyGlobal {...};
```

and

```
[Exposed=AllTheGlobals]
interface GlobalSuperclass {...};

[Global=(AllTheGlobals, GlobalA),
 Exposed=GlobalA]
interface GlobalA : GlobalSuperclass {...};

[Global=(AllTheGlobals, GlobalB),
 Exposed=GlobalB]
interface GlobalB : GlobalSuperclass {...};
```

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